Check engine light only on at 80+ mph speeds

05 Mazda Miata 1.8L 4 cylinder non turbo87k miles, automaticCheck engine would turn on as I would drive 80+ on the freeway/ highway. After a day or two, the light would disappear until I reached speeds that high again. During so, the car's front end would vibrate/ rattle.I got the codes checked out and it turned out to be a spark plug/wires fix. I replaced the spark plugs and everything seemed fine.Same thing happens again when I made a 2 hour trip one day but the engine light would go off the day after. It comes on again, same issue, got the code checked out, same thing.Do I need to replace the wires now and check for a bad plug?Also, what explains the shaking at 80+ mph speeds?Thanks in advance via /r/cars http://ift.tt/2tzS7rX